This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a scanning thermal-inkjet machine and method that construct text or images from individual ink spots created on a printing medium, in a two-dimensional pixel array. The invention employs print-mode techniques to optimize image quality.
(a) Spatial-frequency effects in bandingxe2x80x94A persistent problem in incremental printing is conspicuously visible banding or patterning, which arises from a great variety of causes. Generally these causes are associated with repetitive phenomena that are inherent in the swath-based natured of such printing.
Joan Manel Garcia, in U.S. utility-patent applications Ser. No. 09/150,321 through ""323, particularly addresses problems of patterning in the lateral or transverse dimension, i.e. parallel to the scan axis. He points out that such patterning is especially objectionable when it occurs at spatial periodicities to which the human eye is particularly sensitive.
Garcia shows that such banding can be rendered very inconspicuous at normal reading distances by moving its periodicity to roughly 3 cm (1 inch), or preferably a bit longer. This can be accomplished by tiling printmasks of those widths.
Unfortunately that technique is not now readily applicable to the longitudinal dimensionxe2x80x94i.e. to the direction parallel to the print-medium advance axis. The reason is that, generally, largest current-day printheads are only about 2xc2xd cm (1 inch) long in that direction.
Within the corresponding available range of spatial frequencies, banding in the lower three-quarters of that range (used in single-pass through four-pass printmodes) is quite conspicuous. Unfortunately the current trend toward reducing the number of passes used for printing each image segmentxe2x80x94to enhance overall printing through-putxe2x80x94militates toward use of precisely that part of the range.
(b) Swath-interface effectsxe2x80x94Some banding along the print-medium advance axis arises at the interfaces between swathsxe2x80x94due to the advance errors and xe2x80x9cPADxe2x80x9d errors mentioned above, and due to ink-media interactions such as coalescence or print-medium expansion. Earlier documents such as Doval""s have pointed out that repetitive, small failures of abutment themselves introduce banding (though extremely tiny imprecisions or variations in abutment can be helpful).
Swath-abutment irregularities may represent the single most conspicuous form or type of banding effect. When one swath edge is closely abutted to another, the abutment is almost always imperfectxe2x80x94leading to either a shallow gap between swaths or a shallow overprint where they overlap.
Also the two swaths are generally not exactly the same in darkness or color saturation, adding another element of contrast along the interface. Such problems are aggravated by a high or abrupt gradient of wetness along the edge of a just-deposited swath, when an abutting swath is formed soon after.
(c) Internal effectsxe2x80x94Not all banding problems, however, occur at swath boundaries. Some result simply from nozzle PAD problems and these can be entirely internal to the swath.
Internal patterns can be formed by repetitive coincidences of nozzle irregularities. Prior systematic procedures placed particular irregularly-performing pairs (or other groups) of printhead elements into conjunction with respect to the printing mediumxe2x80x94over and over.
As an example, the Hewlett Packard Company printer product known as the Model 2000C uses two-pass bidirectional printmodesxe2x80x94each pixel row being printed by two separate nozzles. At 24 rows per millimeter (600 dots per inch, dpi), a 12.7 mm (half inch) pen, has 300 nozzles.
Ordinarily nozzles number 1 and 151 contribute drops to the same image rowxe2x80x94using a 6⅓ mm (quarter inch) advance and, again, a two-pass, 300-nozzle printmode. Every 6⅓ mm these same two nozzles are paired (see FIG. 7 and the Table).
If nozzles 1 and 151 when used in combination form a noticeable band effect, this effect is highly visible to the userxe2x80x94because it is present in a repeating pattern, roughly every 6 mm or quarter inch. For example, if both nozzles happen to be directed well away from their nominal target pixel row, then that pixel row will appear unprinted (at least in the particular color in which the head in question prints), rather than the nominal double-printed.
Another kind of band effect can be caused by an interaction of nozzles that are adjacent or nearby. For example assume that nozzle number 5 is aimed xe2x80x9clowxe2x80x9d (toward the nominal target row for nozzle 6). If nozzle 6 is aimed accurately, its target row will be double-printed.
If in addition nozzle 156 is also aimed accurately but nozzle 157 is aimed xe2x80x9chighxe2x80x9d (i.e. both toward the target row for nozzle 156), then in the printed image the common pixel row for nozzles 6 and 156 will be quadruple-printedxe2x80x94while the adjacent rows above and below will each be single-printed rather than the nominal (double printed).
In short, banding within swaths results from repetitive coincidences between irregularly printing elements within each combination. Patterning arises from repetitive, systematic operation.
Objectionable patterning is subject to quantitative effects. Thus some printmasking approaches to patterning in effect simply dilute repetition within an environment of a greater number of alternative states.
(d) Multipass printmode solutionsxe2x80x94Heretofore a common strategy for dealing with all these problems has been to increase the number of passes used to print each image segment. This strategy, however, degrades printing throughput.
It is therefore disadvantageous in the present market, which is increasingly more demanding. This marketplace is characterized by continuously escalating consumer perceptions of what constitutes an acceptable overall image-printing time.
(e) PAD factorxe2x80x94Another kind of band effect arises, particularly with certain pens using tape automated bonding (xe2x80x9cTABxe2x80x9d) nozzle arrays, in image areas where adjacent swaths nominally abut. These effects occur because some modern pens are subject to a concentration of aiming errors at the ends of the penxe2x80x94most classically outboard-aimed nozzles 91 (FIG. 8) as distinguished from the great majority of more centrally disposed nozzles 90.
This higher density of errors, with systematic outboard aim, results from the greater difficulty of maintaining TAB-tape nozzle arrays planar, in comparison with the metal nozzle plates used earlier. In some heads, particularly at the ends of the array, the tape is typically wrapped around the adjacent ends of the printheadxe2x80x94causing the tape to curl very slightly.
The outboard aim in pens of this type increases 93 the overall dimension of the pixel swath in the print-medium-advance axis, beyond the nominal width 92. Typically this overall increase has been on the order of two or three rows.
As a result, when adjacent swaths 94, 96 that should neatly abut are printed with a nominal advance of the print-medium-advance mechanism (FIG. 9, left-hand xe2x80x9cAxe2x80x9d view), those swaths will instead overlap slightly. This occurs because an error region 93 (FIG. 9, xe2x80x9cAxe2x80x9d view) in one of the swaths 94 projects into the region 92xe2x80x2 which should be occupied by the other swath 96.
Meanwhile a like error region 93xe2x80x2 extending from that other swath 96 projects into the region which should be occupied by the first swath 94. As the illustration suggests, these extensions are not limited to the exemplary composite printout 98 of only three swaths 94-96; rather, the phenomenon propagates as at 93xe2x80x3 to still further swaths above and below.
When these swaths are thus printed with nominal advance of the print-medium-advance mechanism, these effects produce, within the composite printout 98, a dark band in each overlap area. The darker inking there is usually at the expense of slight lightening within a few pixel rows inboard from (i. e. above and below) the nominal swath edge. The overall consequence is formation of undesired striations within the composite printout 98.
To mitigate this type of artifact due to outboard PAD error, some printers provide built-in algorithmically operated automatic measurement of the effective increase of the pixel-swath dimension. This is followed by automatic adjustment of the printing-medium advance, typically extending the advance stroke by about half the extension of the swath dimension.
Hence the same swaths 94xe2x80x2-96xe2x80x2 (right-hand xe2x80x9cBxe2x80x9d view, FIG. 9) are now stepped slightly further apart in the longitudinal direction, so that the same error regions 93, 93xe2x80x2xe2x80x94of the alternate swaths 94xe2x80x2, 96xe2x80x2 respectivelyxe2x80x94now either abut or overlap just slightly. The result is a lengthened composite printout 98xe2x80x2 in which at least the conspicuousness of the striations is significantly suppressed.
The measurement is sometimes couched in terms of finding a so-called xe2x80x9cPAD factorxe2x80x9d, the ratio of actual to nominal swath dimensionxe2x80x94in early systems always a number just slightly larger than unity. This technique cures neither PAD nozzle errors nor swath-dimension expansions, but rather accommodates these defects to reduce conspicuousness of overlap.
More recently, with continuing efforts to control PAD error, such error is no longer always outboard and the swath-dimension change is no longer always an expansion but sometimes a contraction. Through the automatic accommodations just discussed, therefore, sometimes the PAD factor is just under unity rather than just overxe2x80x94and the print-medium advance stroke is shortened rather than lengthened.
Finally, in the most-current products PAD error is no longer systematically concentrated at the ends of the nozzle array but rather is somewhat randomly distributed along the array length. With these latest developments the PAD factor differs only insignificantly from unity and the automatic control algorithm, though factory installed and in some units actually operating in the field, usually serves little purpose.
To the extent that dot-placement error is localized randomly along the printhead, the algorithm does not produce the intended results. Furthermore the printout reabeth mains susceptible to the other banding problems introduced in the preceding subsections (a) through (d).
As there remains a very real possibility of future production-run variations reintroducing the desirability of automatic monitoring and stroke adjustment, this algorithmic monitoring and control of effective array length is probably best retained in printer products. It has not heretofore been suggested, however, that this built-in feature might have additional utilityxe2x80x94previously unappreciatedxe2x80x94for addressing the other types of banding phenomena discussed in subsections (a) through (d) above.
(f) Conclusionxe2x80x94Thus failure to effectively address problems of banding in printmodes using low numbers of passes has continued to impede achievement of uniformly excellent inkjet printingxe2x80x94at high throughput. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.
The present invention introduces such refinement. Before proceeding to a relatively rigorous introduction of the invention, this section first presents an informal orientation to some insights which may in a sense have been a part of the making of the invention.
To make banding effects less conspicuous, the spatial frequency or wavenumber of the banding can be raised (i. e. the period shortened, lowered). Banding at higher spatial frequency is less visible to the human eye than banding at a low frequency.
Garcia""s previously mentioned technique works because the visual response characteristic Peaksxe2x80x94so that low frequencies, too, are less visible. For the ranges currently available with printheads 2xc2xd cm long, and less, however, what is most effective is to resort to the higher frequencies.
Some patterns, as noted earlier, are formed by repetitive coincidences of nozzle irregularities. Such undesired coincidences can occur consistently only if common step distances are used repetitively.
Repetitive use of step distances has the effect of placing particular irregularly-performing pairs or other groups of printhead elements into conjunction with respect to the printing mediumxe2x80x94again and again. The coincidences themselves are always present, at least in a latent or virtual sense, because the pairs or groups of irregularly performing elements are always present in the printheadxe2x80x94but they become visible and thereby objectionable only when developed on the printing medium by regular repetition of step distance.
As to the previously mentioned problems associated with abutting swaths, these can be mitigated very greatly by avoiding all formation of abutting swaths. Advantageously this is done with great care, because earlier work such as Doval""s has pointed out that repetitive, small failures of abutment themselves introduce banding. Spacing swath edges away from one another, howeverxe2x80x94or preferably well away, and preferably in a time-varying fashionxe2x80x94very significantly reduces abutment-related banding constituents.
In general the innovations introduced in this document achieve valuable reduction in banding without resort to large numbers of passes. In this way the invention moves the field of incremental printing forward by enabling high image quality without degradation of printing throughput.
With the foregoing preliminary observations in mind, this summary now moves on to somewhat more-formal discussion of the invention.
In preferred embodiments of its first major independent facet or aspect, the invention is a method for printing an image. Throughout this document, it is to be understood that an xe2x80x9cimagexe2x80x9d can be essentially any type of imagexe2x80x94including but not limited to text, computer-aided design (CAD) drawings, and photograph-like pictures. The method includes executing plural passes of a printhead over a printing medium, each pass forming a swath of marks on the medium.
Also included isxe2x80x94between printing passes of the printheadxe2x80x94stepping the printing medium by a nonzero step distance that varies as between steps. The foregoing may represent a description or definition of the first aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, varying the step distance tends to break up patterns otherwise formed by repetitive coincidences of printing-element (e. g. nozzle) irregularities. Such undesired coincidences can occur consistently only if common step distances are used repetitively.
Repetitive use of step distances has the effect of placing particular irregularly-performing pairs or other groups of printhead elements into conjunction with respect to the printing mediumxe2x80x94again and again. The coincidences themselves are always present, at least in a latent or virtual sense, because the pairs or groups of irregularly performing elements are always present in the printheadxe2x80x94but they become visible and thereby objectionable only when developed on the printing medium by regular repetition of step distance.
Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the step distance varies at substantially every step.
In one satisfactory way of operating, preferred for its simplicity, the step distance substantially alternates between two distinct values. In this situation preferably the number of passes is three; and the two distinct values are one-sixth and one-half of a height of the swath.
Another preference is that the number N of passes be odd, and the step distance varies among values having a form (2nxe2x88x921)/2N, where n is an integer ranging from 1 through N. The point here is that use of the invention to disrupt patterning has a quantitative character.
Alternation, for instance, between two distinct values is better than no variation at allxe2x80x94but not as good as rotation among, say, five distinct values, or seven. Thus patterning is subject to a kind of dilution effect, in which conspicuousness can be suppressed more effectively by forcing the patterning to be progressively more complicated.
Yet another preference is that banding effects produced by said method have substantially twice the spatial frequency of banding effects produced using the same number of passes but with nonvarying step distance. Techniques for obtaining this preferred condition are set forth below. This preference represents a different and more sophisticated kind of quantitative strategy: rather than simply brute-force numerical dilution, this preference invokes what might be calledxe2x80x9csmart dilutionxe2x80x9d, which specifically aims to produce a kind of patterning to which the human eye is less responsive.
A still further preference is that substantially no two swath edges coincide. Another kind of preference is that the stepping includes using a step distance that is substantially random or randomized.
Some printers in which the invention can be used have an installed algorithm for accommodating print-medium-advance-axis errorxe2x80x94as set forth for example in the first Doval document mentioned earlier. If the method invention is practiced in such a printer, then preferably the stepping includes using an adaptation of the error-accommodating algorithm.
In preferred embodiments of its second major independent facet or aspect, the invention is apparatus for printing an image on a printing medium. The apparatus includes a printhead.
It also includes some means for passing the printhead over the medium multiple times. For purposes of generality and breadth in discussing the invention, these means will be called simply the xe2x80x9cpassing meansxe2x80x9d. Each pass forms a swath of marks on the medium.
The apparatus further includes some means for spacing edges of each swath away from edges of substantially each other swath, so that substantially no two swath edges coincide on such medium. Again for breadth and generality these means will be called the xe2x80x9cspacing meansxe2x80x9d.
The term xe2x80x9csubstantiallyxe2x80x9d is included here twice, to clarify that this second facet of the invention encompasses apparatus having occasional or unimportant departures from the stated conditions. For instance, a competitor may wish to attempt to avoid the sweep of the present invention by refraining from spacing edges of each swath from edges of other swaths.
More specifically, such a strategy might include allowing two swath edges to coincide from time to time. The term xe2x80x9csubstantiallyxe2x80x9d makes plain that such variations are within the scope of certain of the appended claims, and do not offer an escape from the status of infringer.
The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, avoiding superposition of different swath edges very greatly reduces the single most conspicuous form or type of banding effect. When one swath edge is closely abutted to another, the abutment is almost always imperfectxe2x80x94leading to a shallow gap between swaths or a shallow overprint where they overlap.
Also the two swaths are generally not exactly the same in darkness or color saturation, adding another element of contrast along the interface. Conspicuousness is therefore reduced simply by spacing of the edges apart along the advance direction.
Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the spacing means further include some means for modifying a spatial frequency of banding effects produced by the apparatus.
Another preference is that the spacing means include some means for spacing the edges of swaths from each other by a distance that is substantially random or randomized. Still another preference obtains in case the printing apparatus includes an installed algorithm for accommodating print-medium-advance-axis error; in this event the spacing means include means for adapting the error-accommodating algorithm to space the swath edges well away from each other.
From the foregoing it will be clear that the distance by which swath edges are spaced apart can be a lot or a little. Preferably, however, the spacing means space the swath edges well away from each otherxe2x80x94namely, at least one-twentieth of the swath dimension in a direction of printing-medium advance.
That is to say, the swath dimension under consideration here is the dimension along the direction of printmedium advance; and it is this dimension that is being compared with the spacing-apart of swath edges. This swath-edge spacing is even more preferably at least one-tenth of the swath dimension.
In preferred embodiments of its third major independent facet or aspect, the invention is apparatus for incrementally printing an image on a printing medium. The apparatus includes a carriage for reciprocation over the medium.
Also included is a printhead on the carriage for forming, in substantially each certain multiple of a half-reciprocation of the carriage, a fully inked swath of marks on the medium. (For example, what is described may be an N-pass printmode, with the xe2x80x9ccertain multiplexe2x80x9d being N for bidirectional printing or 2N for unidirectional printing.)
The phrase xe2x80x9cfully inkedxe2x80x9d does not mean that ink is actually applied to every pixel, since a particular image typically does not call for a inkdrop dot in every pixel. Rather, for the purposes of this form of the invention xe2x80x9cfully inkedxe2x80x9d simply means that all pixels have been inked to the extent that they are supposed to be, for the image involved.
Another way to describe this is to say that the swath has been fully addressed. Based on this discussion it is believed that people skilled in the art will understand what is intended. Each swath has at least one region.
The printhead includes multiple individual printing elements. A number of combinations of groups of the elements are used for printing each region of each swath.
The apparatus also includes some means for increasing the number of combinations used for printing each region. For reasons suggested earlier these means will be called the xe2x80x9cnumber-increasing meansxe2x80x9d.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, increasing the number of combinations strongly dilutes the impact of repetitive coincidences between irregularly printing elements within each combination. This is discussed earlier, in regard to the third preference for the first main aspect of the invention.
Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the certain multiple of a half-reciprocation is one half-reciprocation; other preferred values are one full reciprocation and two full reciprocations.
Another preference is that the apparatus further include an advance mechanism for providing relative motion between the carriage and the medium, in a direction substantially orthogonal to the reciprocation. With the advance mechanism in this case is at least one processor for automatically stepping the advance mechanism, generally stepping it once for each half-reciprocation.
Furthermore in this case the number-increasing means include some means for operating the stepping means by a step distance that varies as between steps. Yet another preference in this same case is that the stepping-means operating means include at least one part of the at least one processor.
It is also preferred that substantially no two swath edges coincide, and that the step distance vary at substantially every step (preferably at least substantially alternating between two distinct values). Another preference is that banding effects produced by said apparatus have substantially twice the spatial frequency of banding effects produced using the certain multiple of a half-reciprocation but with nonvarying step distance.
A still further preference is that the certain multiple of a half-reciprocation of the carriage over substantially every portion of such medium be three; and if so that the two distinct values be one-sixth and one-half of a height of the swath. A final preference for mention here is that the certain multiple N of a half-reciprocation be odd; and that the step distance vary among values havingxe2x80x94as beforexe2x80x94the form (2nxe2x88x921)/2N, with n an integer ranging from 1 through N.
In preferred embodiments of its fourth major independent facet or aspect, the invention is a method for printing an image on a printing medium. The method includes executing plural passes of a printhead over a printing medium.
Each pass forms a swath of marks on the medium. The method also includesxe2x80x94between printing passes of the headxe2x80x94stepping the printing medium by a step distance that is substantially random or randomized.
The foregoing may represent a description or definition of the fourth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, random influence helps to further disrupt objectionable patterning that arises from repetitive, systematic operation. As previously pointed out, objectionable patterning is subject to quantitative effects, and even sheer numerical dilution is helpful. Such dilution, however, is very greatly enhanced when the plural different step distances occur randomlyxe2x80x94or at least in a substantially random, or randomized, wayxe2x80x94rather than according to any systematic temporal or spatial pattern.
These objectives, however, are not the only goals encompassed within this fourth facet of the invention under discussion. It is also within the scope of this aspect of the invention to simply wish, for instance, to inject some xe2x80x9cnoisexe2x80x9d into the operation of the system.
There are various reasons for such a strategy. Merely by way of example, the earlier-mentioned patent documents of Garcia have pointed out that a balance between noisiness/graininess and determinism/regularity in an image is one of the general tools of the printing-system designer.
Although the fourth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. Generally such preferences are the same as or analogous to those mentioned above for the first three main facets of the invention.
Thus in particular, if the method is practiced in a system that is subject to printing-medium-axis directionality errorxe2x80x94and especially if at least some amount of that directionality error is not systematically distributedxe2x80x94then preferably the stepping includes adapting a directionality-error-accommodating algorithm. The algorithm provides the substantially random or randomized step distance, for mitigating whatever amount of the directionality error is not systematically distributed.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: